Conventional communications satellites receive and amplify a number of signals from ground based transmitters and direct the amplified signals to receivers in specific geographical locations. This function is commonly referred to as a transparent radio.
Frequency repeater. Transparent repeaters do not demodulate the signals but merely amplify and redirect them. Those carried on satellites are used for commercial applications such as TV broadcast and two-way point to point data links. Signals are sent to the satellite in one frequency band (the uplink) and are redirected to user terminals in a different band (the downlink).
A typical uplink and downlink might comprise several channels from a total up to 100 within a 2 GHz band. The uplink channels are amplified, changed in frequency to fit within the downlink band and directed to a particular geographical location via a high gain antenna. A simplified example of this is shown schematically in FIG. 1. The downlink band 1 is shown as comprising only 32 channels with exaggerated guard band spacing for clarity. Each of the channels shown in this example could accommodate several narrowband channels in practice. Each of the downlink beams 2, 3 is shown as selecting 8 discrete channels from the downlink band.
Modern communications satellites are required to support a variety of business models, which may change over time. Satellite operators require their costly satellite infrastructure to be operationally flexible in terms of channel to beam connectivity, which implies the need for complex RF switch and filter networks that can be reconfigured easily.
These networks have been implemented typically using electromechanical switches but in the low power signal section of the satellite repeater they may now be realised using wide-band solid state switch matrices or digital signal processing techniques. Such input networks are able to determine the correspondence between uplink and downlink channels (frequency flexibility) but the amplified signals then have to be assembled in groups that can be carried within a particular antenna beam (channel to beam flexibility).
In a conventional repeater, as shown in FIG. 2, low power input filters ensure that each amplifier carries typically just one channel. All channels are then amplified to a similar power level and conveniently, by restricting the amplifiers to a narrow band of frequencies, the amplifier power outputs can be optimised. The principle of operation for a typical repeater scheme with a degree of channel to beam flexibility is shown in FIG. 2. For clarity this is limited to the selection of any three outputs from any five amplifiers. In this scheme an input signal network or processor 21 containing low power input filters directs each of the selected downlink channels to an amplifier 22. High power microwave selector switches 23 then direct the selected channels, via a power combiner 24, to the antenna feed 25 that corresponds to the desired geographical coverage area. A second beam 26 requires a similar switch and filter arrangement and this would be repeated for each separate beam required. The combiner achieves high power transfer efficiency by incorporating filters that correspond to the channels that may be selected. It is clear from this architecture that for satellites that may be required to operate with many possible combinations of channels and coverage areas, a large number of high power selector switches and output filters is required. This adds mass and cost to the satellite but there is the advantage that such a scheme may obviate the need for additional switches to accommodate equipment failures.
The complexity of combining the outputs from several different amplifiers can be avoided if a Wide-band Multi-Port Amplifier (WMPA) is used. A WMPA comprises a set of wide-band amplifiers that are connected via microwave hybrids. This arrangement ensures that signals presented at a specific input port pass equally through all of the amplifiers and the combined outputs are routed to the corresponding output port. The principle of operation for a two beam case is shown in FIG. 3. An input signal network or processor 31 directs a number (n) of the selected downlink channels to a WMPA 32. The WMPA comprises a number (N) of amplifiers that could operate over the whole of the downlink band (typically 2 GHz wide), where the number N is not necessarily equal to n. This scheme avoids the need for high power switch networks because the channels required for a specific coverage or beam are combined in the low power input section and map across to the corresponding amplifier output port. A 2 GHz band-pass filter 33 ensures any “out of band” products are not carried through to the feed 34. Advantageously, this arrangement also confers a high degree of tolerance to individual amplifier failures when the number of amplifiers is large. Principal disadvantages of MPAs, when operating over a wide bandwidth, are the difficulty of ensuring adequate inter-port isolation (the cross-talk problem) and achieving high overall amplifier efficiency (RF output vs. energy input).
An alternative method of avoiding the complexity of combining the outputs from several different amplifiers is to perform the power combination in free space. This is achieved by using a multi element phased array or active antenna. In this arrangement each antenna element is driven by a dedicated low power wide-band amplifier and all of the amplifiers carry all of the downlink signals. The principle of operation is shown in FIG. 4.
The input signal processor 41 determines the relative amplitude and phase of the signals presented to each of the amplifiers 42 and their dedicated antenna feeds 43. The amplitude and phase relationships are determined such that the selected downlink channels appear in the appropriate antenna beam 44.
Advantageously, this arrangement also confers a high degree of tolerance to individual amplifier failures, since the number of antenna elements and hence the number of amplifiers is typically large. Unfortunately, this solution simply transfers the complexity problem to the signal processor, which for some applications may not be practical to build using current technology.